Potatoes and other tubers are typically held in large storage facilities after harvest to maintain quality and provide a uniform flow of product to market. Modern storage facilities can hold more than 20,000 tons of potatoes in piles as high as 25 feet. Temperature and humidity must be controlled during storage to prevent excessive dehydration, decay, and the development of high sugar concentrations. Most table stock potatoes, for example, are typically stored at about 40-45° F. and about 95% relative humidity, while potatoes for chips or French fries are typically stored at about 45-55° F. and about 95% relative humidity.
Sprouting can be expected in most potatoes after a couple of months of storage. Although limited sprouting does not seriously damage potatoes for food purposes, heavily sprouted potatoes tend to shrivel and can be difficult to market. Accordingly, successful long-term storage generally requires the use of a sprout inhibitor in combination with proper storage conditions.
Isopropyl-N-chlorophenylcarbamate (CIPC) is a very effective post-harvest sprout inhibitor registered for use in the United States. CIPC has the consistency of solid paraffin wax at temperatures less than about 105° F., and it is typically sold in this form for ease of handling and storage. For application, solid CIPC is melted to form a liquid, and then applied to potatoes or other vegetables as an aerosol or emulsifiable concentrate. Clove oil (sometimes marketed as Biox-C), propylene glycol, methanol, and/or other solvents are often mixed with CIPC so that it remains liquid at ambient temperatures (e.g., temperatures less than about 105° F.) before application. Such CIPC mixtures typically contain about 78% or less of CIPC by weight, with the remainder of the solution consisting mostly of solvent.
U.S. Pat. Nos. 5,935,660 and 6,068,888 disclose methods for applying substantially pure CIPC aerosols to potatoes in storage facilities. More specifically, these patents disclose methods in which solid CIPC is melted in a heated reservoir to form liquid CIPC at the job site. The liquid CIPC is then conveyed through a heated and insulated conduit to an aerosol-generating device positioned next to the storage facility. The aerosol-generating device forms an aerosol of microscopic CIPC crystals, and directs the aerosol through a duct that extends into the storage facility. Large fans inside the storage facility drive the aerosol into a plurality of perforated ducts extending beneath the pile of potatoes. As the aerosol flows out of the perforations and moves upwardly through the pile of potatoes, the CIPC crystals are deposited on the potatoes.
The '660 and '888 patents cite a number of advantages to using substantially pure CIPC aerosols for potato treatment. For example, these patents note that solid CIPC is easy to handle and transport. In addition, they note that the use of substantially pure CIPC aerosols (as compared to, for example, CIPC mixed with alcohol solvents) eliminates the introduction of alcohol, alcohol combustion products, and/or alcohol decomposition products into the storage facility.
There are, however, a number of shortcomings associated with the methods described in the '660 and '888 patents. For example, these methods require a heated reservoir, a heated and insulated conduit, and other special equipment to maintain the molten CIPC at a temperature of at least 105° F. prior to and during application. Another shortcoming of this approach is that the CIPC crystals in the aerosol tend to collect on the fan blades and other surfaces of the air circulation system in the storage facility. As a result, a significant portion of the CIPC is wasted and ineffective. Furthermore, the CIPC buildup must be cleaned off the air handling equipment periodically.
U.S. Pat. No. 4,887,525 discloses a method of CIPC application that purportedly reduces the amount of CIPC waste. More specifically, this patent discloses the use of a frequency drive to reduce fan speed and slow the flow of CIPC aerosol to less than 5 Standard Cubic Feet per Minute (SCFM). Apparently, reducing the flow rate to less than 5 SCFM reduces the loss of sprout inhibiting chemical by agglomeration and impingement with the surfaces of the air handling system.
One shortcoming of the method taught by the '525 patent is that achieving an even distribution of CIPC throughout the potatoes is difficult with air flows of 5 SCFM or less. This difficulty is increased when the CIPC is mixed with clove oil or other solvents, because the resulting aerosol particles are heavier than CIPC alone. Another shortcoming of this approach is that it tends to increase the amount of time necessary to apply CIPC to stored potatoes.